About Shigella

From the nation’s leading law firm representing victims of Shigella and other foodborne illness outbreaks.

Chapter 7

How is Shigellosis Diagnosed?

Shigella infection is diagnosed through testing of a stool sample.

Because the symptoms of a Shigella infection are consistent with a fairly large number of potential illnesses, including most foodborne infections, a diagnosis must be confirmed by a laboratory test. [5, 11, 26] First a stool sample must be obtained from the potentially infected person, and then the sample is placed on a medium to encourage the growth of bacteria. If and when there is growth, the bacteria are identified, usually by looking at the growth under a microscope. [20, 26]

The laboratory can also do special tests to tell which species of Shigella the person has, and which antibiotics would be best to treat the infection. [16, 22, 30] Antibiotic-sensitivity tests are important because Shigella is often resistant to multiple antibiotics. [16, 30]

More advanced testing and surveillance methods, such as plasmid profiling and chromosomal fingerprinting, can also be used. [11, 20, 29] So-called “genetic fingerprinting” of the bacterial isolate, using pulsed-field gel electrophoresis (PFGE) is a molecular technique that can help to characterize Shigella isolates, whether obtained from human or food samples. [11, 27] Taken together, all of these tests can assist public health officials in determining whether cases (confirmed infections) are isolated or associated with common-source outbreaks. [19, 20, 27]

Efforts to identify outbreaks of foodborne illness—whether caused by Shigella or other pathogens—are important to preventing the secondary spread of infection, especially with bacteria as highly communicable as Shigella. [1, 11, 21] One major advance in these efforts was the creation of FoodNet, an active surveillance system for foodborne illness. As described by the CDC:

FoodNet workers regularly contact more than 300 laboratories for confirmed cases of foodborne infections in 10 states encompassing a population of more than 44 million persons. In addition to monitoring the number of Shigella infections, investigators monitor laboratory techniques for isolation of bacteria, perform studies of ill persons to determine exposures associated with illness, and administer questionnaires to people living in FoodNet sites to better understand trends in the eating habits of Americans. [11]